1. Field of the Invention
The present invention relates to a structure of a liquid accommodation container for accommodating ink and the like, and more particularly, to an ink tank of a recording apparatus on which a cartridge, which includes a recording head for executing recording according to an ink jet system and the ink tank for supplying ink to the recording head, is mounted.
2. Description of the Related Art
There has been known an arrangement disclosed in, for example, Japanese Patent Application Laid-Open No. H8-112907 as a method of detecting a remaining amount of ink or the presence or absence of ink in an ink tank (liquid accommodation container) for accommodating a liquid such as ink and the like, that is, as a method of optically detecting the presence of ink. Japanese Patent Application Laid-Open No. H8-112907 discloses an ink jet recording apparatus employing a method of causing light to pass through a part of a wall surface of a light transmitting ink tank and detecting the change of a light reflectance in an interface between the wall surface and a negative pressure generation member in order to detect a remaining amount of ink in an ink tank having the negative pressure generation member such an absorbent, a foaming agent, and the like and.
Japanese Patent Application Laid-Open No. H7-218321 discloses an ink tank having an optical ink detector which is formed of the same light transmitting material as that of an ink tank and the interface of which to ink has a predetermined angle with respect to a light path.
Japanese Patent Application Laid-Open No. H9-29989 discloses an ink jet recording apparatus that can detect the presence or absence of ink and the presence or absence of an ink tank by a set of an optical sensor using an light emitting device and a light receiving device.
Further, Japanese Patent Application Laid-Open No. 2002-321388 discloses an arrangement that a member, which has two slanting surfaces forming a predetermined angle is disposed on one of walls constituting an ink tank at a position where the member is in contact with ink. In the above arrangement, an amount of ink can be securely detected by changing an amount of light, which is irradiated to the ink tank from the outside thereof and reflected to the outside, by the two slanting surfaces depending on an ink absence state and an ink presence state.
The two slanting surfaces described above are conveniently called a prism portion in the following description because they optically function as a so-called prism when ink is absent, and an ink amount detection system employing light reflected by the prism portion is conveniently called a prism detection mechanism in its entirety in the following description.
FIGS. 7A and 7B show a conventional embodiment of an ink tank on which a prism is mounted, wherein FIG. 7A is a sectional view showing an overall arrangement of the prism, and FIG. 7B is a sectional view taken along the line 7B—7B in FIG. 7A.
The ink tank is a liquid accommodation tank having a negative pressure generation member accommodation chamber 14, a liquid accommodation chamber 17, and further a prism 20. The negative pressure generation member accommodation chamber 14 accommodates a negative pressure generation member 11 as well as includes a liquid supply port 12 and an atmosphere communication unit 13, the liquid accommodation chamber 17 has a communication unit 15 for communicating with the negative pressure generation member accommodation chamber 14 as well as forms an substantially hermetically sealed space, and the prism 20 detects the presence or absence of a liquid 16 accommodated in the liquid accommodation chamber 17.
A prism detection mechanism composed of the prism 20 shown in FIGS. 7A and 7B will be explained in detail with reference to FIG. 8.
FIG. 8 is a view showing an example of a positional relationship among the light transmission type prism 20 disposed on the bottom 21 of the ink tank, a light emitting device 22 for irradiating light to the prism 20, and a light receiving device 23 for receiving the light.
As shown in FIG. 8, the prism 20 is molded integrally with the ink tank and disposed on the bottom 21 thereof. Although the ink tank has a basic wall thickness of 1.7 mm to 2.0 mm, the prism 20 has such a sectional shape that the 90° apex thereof protrudes into the ink tank by a height of 3.2 mm (FIG. 7B).
The light emitted from the light emitting device 22 is incident on the prism 20 from below a lower portion of the ink tank.
When the ink tank is filled with a sufficient amount of ink so that the ink is in contact with the surface of the prism 20, the light incident on the prism 20 is absorbed into the ink through light paths (1) and (2′) and does not return to the light receiving device 23. In contrast, when the ink in the ink tank is consumed in such an amount that it is not in contact with the surface of the prism 20, the incident light is reflected by a slanted surface of the prism 20 that acts as the interface thereof with the ink and reaches the light receiving device 23 through the light path (1) and light paths (2) and (3) as shown in FIG. 8. Presence or absence of the ink is detected by whether or not the light emitted from the light emitting device 22 returns to the light receiving device 23.
Note that the light emitting device 22 and the light receiving device 23 are ordinarily disposed on a recording apparatus body side.
Further, in an actual system, a threshold value is provided in consideration of the effect of background light, and the like, and presence or absence of ink is ordinarily determined depending on whether or not the threshold value is exceeded.
Japanese Patent Application Laid-Open No. H10-323993 proposes an example of an arrangement for coping with the effect of background light. In the arrangement, a recess is formed on the bottom, which corresponds to a prism, of an ink tank in order to improve a drawback that presence or absence of ink cannot be accurately detected. The drawback occurs because the light emitted from a light emitting device is directly reflected on the bottom of the ink tank, on which the prism is disposed, and thus the light receiving device detects the light in an amount larger than an amount which is supposed to be received. According to the arrangement, an amount of the emitted light, which is reflected by the bottom and directly reaches the light receiving device, is reduced because the reflected light path of the emitted light is disturbed by the recess, thereby the effect of the reflected light to the light receiving device can be decreased.
It can be said that the prism detection mechanism explained above is a very reasonable method as a method of detecting a level or presence or absence of the ink in an ink tank.
In recent years, however, it is required to provide a high quality ink jet recording apparatus at low price.
Ink amount detection means including the prism detection mechanism warns run out of ink which occurs sometimes and does not determine the fundamental capability of a recording apparatus such as print quality, a print speed, and the like. However, provision of the ink amount detection mechanism is very effective to save ink, sheets, and time by preventing reattempt of printing due to the run out of ink.
As a result, the ink amount detection mechanism has been supported by users because they recognize the importance thereof.
In contrast, it is true that the ink amount detection mechanism has a significant influence on the manufacturing cost of a recording apparatus and an ink tank because it requires a precise mechanism and the number of parts is increased, and thus the quality of a manufacturing processes must be enhanced. This problem will be explained below in detail.
Ordinarily, although a light emitting unit and a light receiving unit is commercially available as an integral part, a light sensor unit composed of a combination of independent parts may be employed.
In any of the cases, the light receiving unit and the light emitting unit are fixed with a predetermined interval set therebetween, and the interval is conveniently called a lens interval 30 of a light sensor.
As shown in FIG. 9, a first slanting surface 31 of two slanting surfaces of a prism of an ink tank confronts the light emitted from a light emitting device 22 at an angle of 45°, and a second slanting surface 32 is formed at an angle of 90° with respect to the first slanting surface 31 and confronts the light receiving device 23 at an angle of 45°.
The light emitted from the light emitting device 22 is reflected by the first slanting surface 31 at an angle of 90° and reaches the second slanting surface 32. However, it is important that the distance 33 from a light reflecting portion 36 on the first slanting surface 31 to the intersecting point at which the line from the light reflecting portion 36 intersects a prism center line 35, which is obtained by equally dividing the apex of the prism, at a right angle be equal to the distance 34 from a light reflecting portion 37 on the second slanting surface 32 to the intersecting point at which the line from the light reflecting portion 37 intersects the prism center line 35 at a right angle as far as possible.
An ideal light path from the light emitting device 22 of the sensor to the light receiving device 23 thereof through the prism 20 is called an optical axis.
FIG. 10 shows a case where the center of the light path (optical axis) is deviated right from the prism center line 35 by an amount of offset (deviation) 41. The deviation is not caused intentionally but is an indispensable deviation due to the accuracy of sensor parts, the mounting accuracy of the sensor parts, and the positional accuracy and the mounting accuracy of the prism resulting from the ink tank being mounted.
As shown in FIG. 10, the light emitted from the light emitting device 22 is displaced from the optical axis of the light receiving device 23 due to the occurrence of deviation of the optical axis (optical axis deviation) 42. In a triangular prism as the prism 20, an amount of deviation 42 of the optical axis of the light receiving device is theoretically twice as large as the amount of deviation 41.
Accordingly, to permit the light receiving device 23 to effectively receive light, the intermediate axis of the lens interval 30 must be disposed in alignment with the prism center line 35 more accurately.
When the intermediate axis is deviated from the prism center line 35 as described above, even if the recess is formed on the bottom of the ink tank where the prism is disposed as disclosed in Japanese Patent Application Laid-Open No. H10-323993, this arrangement cannot cope with the deviation because it is not an arrangement for compensating the deviation.
FIG. 11 explains the relationship between the amount of deviation of the light sensor described above and the distribution of intensity of light that reaches the light receiving sensor.
According to the FIG. 11, when the amount of deviation (offset) is zero, that is, when the optical axis of the sensor is perfectly in alignment with the center of the prism 20, the intensity of reflected light has a value greatly larger than the threshold value for determining ink presence and absence.
However, an increase in the amount deviation abruptly decreases an amount of received light, and when the amount of deviation is in the range less than a portion “a” and in the range larger than a point “b” in FIG. 11, the intensity of reflected light has a value less than the threshold value, and thus the ink presence and absence cannot be correctly determined.
Since the ink tank is mounted on a moving portion of an ink jet recording apparatus, in particular, on a carriage in many cases, it must be aligned with a pinpoint accuracy.
Further, the position of the prism must be accurately determined from the position of the ink tank determined with respect to the ink jet recording apparatus.
Further, the prism must be formed in a precise shape because it is an optically precise prism.
To satisfy the above requirements, it is strictly required to assemble the parts and the mechanisms of the prism with a pinpoint accuracy and further it is important to confirm the quality of them, thereby the cost of the prism is inevitably increased.
As ink jet recording apparatuses are miniaturized more and more in recent years, a space, which can be occupied by a prism in an ink tank, becomes very small. Accordingly, when a polygonal prism is employed, one side thereof has a very narrow size, from which an amount of deviation that is allowed to the polygonal prism with respect to an optical axis is actually smaller than that of a triangular prism.
Almost all the ink tanks for the ink jet recording apparatuses are formed of a relatively less expensive plastic by ordinary injection molding. It is very difficult to accurately mold an optical prism having a complex shape as described above by the injection molding. This is because that since the wall thickness of a prism portion is lager than that of other portions, a sink mark occurs in the prism portion because the resin is shrunk after it is molded in the plastic injection molding method. Since the surface shape of the prism portion is made uneven by the sink mark, the optical function thereof is deteriorated by it.
To prevent the sink mark, a space called a relief is ordinarily formed in a molded product, the relief cannot be freely designed because it must be formed at a position offset from an optical path.
Although some plastics are less shrunk in molding and can be selected as materials for overcoming the problem of sink mark described above without the relief, their cost is very expensive.
Further, when a plastic is selected as a material, since the chemical stability of the material to ink must be also taken into consideration, a range of selectable materials is greatly limited.
According to the knowledge of the inventors, an olefin resin is most preferable as a material which has good balance in reasonable price, chemical stability, and moldability as well as permits light to pass therethrough.
Further, a polypropylene resin is most suitable when injection moldability and mechanical rigidity are taken into consideration.
Further, as an another means, a trial for increasing an amount of emitted light was also executed to secure a sufficient amount of light even if an optical axis is deviated with respect to an ink jet recording apparatus.
However, when the amount of emitted light is increased immoderately as shown in FIG. 12, an amount of background reflection 70 is increased by irregular reflection in the ink tank when ink exists therein and background reflected light reaches the light receiving device, from which an error region 71 occurs and an erroneous determination that “ink is absent” is made thereby.
To construct a user-friendly ink jet recording apparatus as described above, it is indispensable to provide a mechanism for detecting an amount of ink remaining in the ink tank with the apparatus, and the optical remaining amount detection mechanism using the prism is excellent from the relationship between cost and reliability.
However, it has been difficult to more reduce the cost of the optical remaining amount detection mechanism using the prism because the number of parts is increased to cope with a requirement for a pinpoint accuracy and the quality thereof must be sufficiently managed.